Effect of Natural Organic Matter on Bioavailability and Mobilization of Metals Project Leader: Dominic M. Di Toro Co-Leaders: Richard F. Carbonaro, Kevin J. Farley and Douglas J. Doren Modern methods for predicting the toxicity of metals at contaminated Superfund sites require that bioavailability be explicitly evaluated. The Biotic Ligand Model (BLM), which accounts for the effects of metal speciation at the site of toxic action - the biotic ligand - is currently being used by the EPA for water column copper criteria and will be the basis for all future criteria. Water column constituents such as inorganic ligands and organic matter (DOC and POC) play an important role in the BLM, competing with the biotic ligand for the metal. The BLM requires a speciation model that can account for partitioning to natural organic carbon, either dissolved (DOC) in the aqueous phase or particulate (POC) in sediments or soils. The most serious drawback to application to Superfund sites is the lack of models for arsenic, As(lll) and As(V), and chromium, Cr(lll) and Cr(VI) species. Our first overall objective is to remedy this situation. The chromate speciation model will be also applied to Dr Wirgin's tomcod toxicity data There is at present no mechanistic model for establishing chromium sediment criteria for Superfund sites. In the presence of reduced sulfur, Cr(VI) is reduced to Cr(lll) which is very insoluble and not toxic in the normal pH ranges found in sediments. However, Cr(VI), which can exist in the aerobic zone of sediments, is quite soluble and toxic. Our second overall objective is the determine the rates of Cr(lll) transformation to Cr(VI) in sediments and to build a model that determines the extent of transformation.